The present embodiments relate to imaging motion with ultrasound. Fluid motion (flow) or tissue motion is detected with ultrasound. A Doppler or correlation process estimates motion parameters such as velocity, energy or variance from received echoes. If the motion is undersampled, such as due to a long pulse repetition interval (PRI), the estimates are aliased. As the PRI decreases, the maximum depth of motion imaging may be limited. If the motion is oversampled, the sensitivity to low-velocity motion may be lost.
The PRI is increased or decreased to provide the desired sampling rate. For example, a user alters the PRI so that flow with the highest velocity in a region of interest or through a sequence is not undersampled. The user may purposefully set the PRI to undersample the highest velocities for sensitivity to low-velocity flow, resulting in aliasing of higher velocities. As another example, the PRI is adaptively updated by a processor in a medical diagnostic ultrasound imaging system. U.S. Pat. No. 6,478,742 discloses adjusting a pulse repetition frequency for automatic optimization. U.S. Pat. No. 7,288,068 also discloses automatic optimization of the PRI. However, optimizing the PRI to prevent aliasing of high-velocity flow may result in undesired decrease in low-velocity motion sensitivity.